Electromagnet



March 10, 1959 P. TROMBETTA 2,877,390

ELECTROMAGNET Filed April 2, 1955 s sheets-sheet 1 7j/l1 f5@ 0/ 24 INVENTOR.

March 10, 1959 Filed April 2, 1955 P. TROMBETTA ELECTROMAGNET @gi/j 3 Sheets-Sheet 2 70 al A INVENTOR. WZ@ WZQQZZ March 10, 1959 P. TROMBETTA 2,877,390

ELECTROMAGNET Filed April 2, 1953 3 Sheets-Sheet 3 V 5g 25 57 QZ ,55 gj I QZ 2Q /4 GV?? 25 65a F1a@ Il@ Q5 i@ 2?: yl @L @afi-2 197 l.

s @a 29h 96 4Q IZ? 0g! *6); 65g 5M@ O, j L27 @@/24 92 INVENToR. TZGWZMQ 'BY United States Patent ELECTROMAGNET Panllo Trombetta, Milwaukee, Wis. Application April 2, 1953, Serial No. 346,405 15'Claims. (Cl. 317-191) The present invention relates to electromagnets of the type which employ a movable plunger which upon reciprocation thereof actuates an element of a suitable mechanism. More specically, the present invention is concerned with an eleetromagnet for use on alternating current which is capable of use in applications where quietness ot' operation is imperative and wherein the electromagnet must be substantially free from the undesirable elects of residual magnetism.

Electromagnets are extensively employed for performing numerous functions in all types lof machinery wherever some control function -must be performed which must be readily controlled. One type of electromagnet employing a reciprocating plunger comprises a stationary magnetic structure of somewhat U-shaped coniguration formed of laminated iron with a central pole piece disposed at the bight of the U for engagement with the reciprocating plunger in one extreme position of the plunger. Such electromagnets have also commonly included a T-shaped plunger, with the head of the T slidably mounted between the arms of the U-shaped magnet, which arms also define a guide means for the plunger. It will be understood that electromagnets should be capable of being mounted in almost any position whether the plunger moves in a vertical direction or in a horizontal direction or some other direction. Obviously, when the plunger is movable in a horizontal direction, it will be apparent that one arm of the head of the T-shaped plunger will, by virtue of gravity, tend to be more closely adjacent the arm of the U-shaped pole piece of the electromagnet than the other, thus producing an unbalance, even though slight, when no wear is involved. As the electromagnet is used, wear will cause this unbalance to increase by a substantial percentage, and in time the unbalance will be sufficient to cause sticking and resultant. rnisoperation of the solenoid or electromagnet. It would be desirable to provide improved plunger guide means for an electromagnet in which the wear occasioned by operation of the solenoid will not have the disadvantageous etect noted above.

it will be understood that electromagnets energized by a single phase alternating current have an inherent problem with respect to noise, since by virtue of the occurrence of a current zero there also occur intervals of zeroy force. Consequently, there is a tendency for the solenoid or electromagnet armature to chatten Means in the form of shading coils have been employed with, single phase alternating current electromagnets to mitigate the pulsations in force between the armature and` pole face. Gbviously, this is ot' importance only whenV the armature is in contact with the pole face, becausev only then do the force pulsations allow the armature to hammer on the polev face and produce the undesirable elect known as chattering It is essential, therefore, that at no time does the force acting on the electromagnet armature drop below the load on the armature when the armature is against its stop, which stop is commonly the pole face. The present invention is concerned with an arrangement in which the armature actually makes a three-surface Contact to insure a maximum force together with suitable means in the form of improved shading coils to insure that at no time does the force acting on the plunger, when in engagement with the stops, fall below the minimum force required sothat absolute quietis obtained.

By virtue of residual magnetism there is a tendency in electromagnets to produce sticking of the armature for a short interval of time following deenergization, and it is desirable to provide an arrangement. in which such sticking or any adverse eiiects of residual magnetism are eliminated.

Accordingly, it is an object of the present invention to provide a new and improved electromagnet.

It is another object of the present invention to provide an improved electromagnet in which there is contactV at three separate surfaces between the movable armature and the stationary magnetic structure.

It is still another object of the present invention to provide an improved electromagnet in which nonmagnetic material is employed between the head of the plunger and the laminated pole pieces to reduce to a minimum any unbalance and to prevent wear from having an appreciable effect on causing unbalance.

It is another object to provide in an electromagnet an improved nonmagnetic plunger guide.

Still another object of the present invention resides in the provision of improved supporting means for holding the electromagnet in assembled condition.

It is a further object of the present invention to provide means for eliminating any undesirable effects with respect to residual magnetism.

It is another object of the present invention to provide an electromagnet with improved shading coils to msure quiet operation under all conditions. v

Further objects and advantages of the present invention will become apparent as the following description proceeds, and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specificanon.

For a better understanding of the present invention reference may be had to the accompanying drawings, in which:

Fig. l is an elevational view of one embodiment of the present invention showing an electromagnet with the plunger or armature mounted for movement along a horizontal axis;

Fig. 2 is a sectional view taken on line 2--2 of Fig. l;

Fig. 3 is a sectional view taken on line 3 3 of Fig. 1;

Fig. 4 is an enlarged sectional view taken on line 4 4 of Fig. 2, assuming that Fig, 2 shows the complete structure;

Fig. 5 is a somewhat enlarged view of Fig. 2 with a portion thereof removed and certain other portions thereof cut away more clearly to illustrate certain features of the present invention;

Fig. 6 is a sectional view taken on line 6--6 of Fig. 5, assuming that Fig. 5 is not in section;

(liig. 7 is a perspective view of a portion of Figs. 5 an 6;

Fig. 8 is a view similar to Fig. 7 showing a modification of this portion which may be substituted for the element shown in Fig. 7; i

Fig. 9 is a similar view tion of this portion which ment shown in Fig. 7;

Fig. 1l) is a somewhat schematic diagram of certain portions of Fig. 5 to illustrate the details of one of the to Fig. 7 showing a modificamay be substituted for the eleasr'aeeo shading coils indicated in Fig. 5 and associated with the plunger of the electromagnet;

Fig. 11 is a .view partly in section of another modiiication of the present invention;

Fig. 12 is a view looking'in the direction of the ai'- rows 12-12 of Fig. 1l;

Fig. 13 is a sectional view taken on line 13-13 of Fig. 1l;

Fig. 14 is a view partly in section showing the electromagnet, illustrating another modification of the present invention, with the winding thereof omitted;

Fig. 15 is a sectional View taken on line 15-15 of Fig. 14, assuming that Fig. 14 shows the complete structure;

Fig. 16 is a view similar to Fig. 14 showing another modification of the present invention; and

Fig. 17 is a sectional view taken on line 17-17 of Fig. 16, again assuming that Fig. 16 shows the complete structure.

Briefly, the present invention is concerned with an electromagnet of the type having a short central pole piece and a divided laminated pole piece on opposite sides of the central pole piece. Integral extensions from each arm of the divided laminated pole piece extend toward each other and toward the axis of the central pole piece. A T-shaped plunger is disposed to have the head of the T engage both of the extensions, and the end of the T to engage the central pole piece, thus providing three engageable contact surfaces when the armature is in one extreme position. The present invention also contemplates the inclusion of nonmagnetic plunger guides which may take numerous forms such as plates of non- Amagnetic material disposed between the head of the plunger and the portions of the laminated pole piece beyond the extensions. Various meansv for holding such nonmagnetic plunger guides in position are disclosed. The present invention further contemplates improved pole shaders to insure a quietly operating alternating current electromagnet. `Additionally, the armature includes a short nonmagnetic gap therein to reduce the undesirable effects of residual magnetism.

Referring now to the drawings, and particularly to Figs. 1 to 7 thereof, there is illustrated an electromagnet generally designated by the reference numeral 20. As illustrated, this electromagnet 20 comprises a stationary field structure comprising a stack of a plurality of U- shaped laminations 21 (Figs. 2 and 3) defining a short central pole piece portion 22 defined at the center of the bight of the U, and arms 23 and 24 disposed on either side of the axis of this central pole piece. As vfurther illustrated, the arms 23 and 24 are provided withlateral extensions 25 and 26, respectively, which extensions are on adjacent sides of the arms 23 and 24. The lateral extensions 25 and 26 are intermediate the ends of the arms 23 and 24. It should be understood that the short central pole piece 22 is not essential, and a portion of the bight of the U may itself define the central pole piece.

It will be understood that the somewhat U-shaped laminations 21 making up the stationary magnetic structure are clamped together by suitable rivets 27. Preferably, also, these laminations are clamped between metallic clamping plates 28 and 29 which may be formed of nonmagnetic material such as copper or brass, the purpose of which will become apparent as the following description proceeds. They may also be formed of magnetic material, however. The magnetic structure may also be provided with suitable openings such as 30 to facilitate mounting of the structure in any suitable manner. Ad-

ditionally and as illustrated in the drawings, a suitable base plate 31 may be brazed or otherwise secured to the bight of the U-shaped magnetic structure. This base plate 31 may likewise be provided with a plurality of openings 32 for mounting purposes. It is common practice for such electromagnets to be used in a large number of different applications, and, hence, the numerous mounting means, such as the openings 30 and 32, are provided to take care of the various applications. Obviously, if the electromagnets were made for a single application, the mounting means would not need to be as extensive and could be designed for the single application. ln the drawings the large number of mounting means are indicated to show universal application.

From the above description it will be apparent that there is defined between the lateral extensions 25 and 26, the central pole piece 22 and the arms 23 and 24, a s pace for receiving a suitable electrical winding for energizing the electromagnet 20. The construction of the electrical winding forms no part of the present invention. As illustrated, however, this electrical winding, generally de signated by the reference numeral 34, is wound on a suitable spool or form 35 having end pieces 35a and 351i and a central spool portion 35C. As illustrated, these spool portions are held together by suitable clamping means 36, best shown in Figs. l and 2 of the drawings. The spool 35 preferably has a rectangular central opening to receive therein the central pole piece 22. it will be apparent that the central pole piece 22 must be relatively short to permit placing the coil in position beneath the extensions 25 and 26, and this central polepiece 22 tends to maintain the spool 35 having the winding 34 thereon iii position, so that it will not tend to slip sideways out of the space defined by the stationary magnetic structure already described.

It will be apparent that the armature or plunger of the electromagnet may take numerous forms, and the plunger may push or pull the mechanism it is to operate. Both arrangements are indicated in the drawings, but, as illustrated in Figs. 1 to 7 of the drawings, .the armature 42 is indicated as an armature made up of a plurality of T-shaped laminations 43, so as to define a T-shaped plunger having a head portion 42a and a main portion 42b. Additionally, in the embodiment illustrated in the drawings, the plunger 42 has a central extension 45, preferably comprising a pair of `nonrnagnetic members clamped between two stacks of laminations 43 defining the plunger, as best shown in Fig. 3 of the drawings. It will be understood that to accommodate the extension 45 the bight portion of the U-shaped magnetic structure is provided with a suitable opening 46 therein, which is actually disposed in the center of the central pole piece 22. It will be understood that the extension 45 may be provided with a suitable opening 47 for connection to any operating mechanism lor device to be operated. It should, furthermore, be understood, however, that the extension 45 may be dispensed with and a connection to the plunger made in some other manner, as in the electromagnets shown in Figs. 14 and 16. Preferably, also, the plunger 42 includes side plates 49 and 56 yof metallic material to define the outside surfaces of the plunger. Suitable rivet means 51 hold the plunger in assembled relationship, including the side plates 49 and 50, the two stacks of laminations 43, and the plates at the center defining the extension 45. Under certain conditions it may also be desired to provide the plunger 42 with a hardened plate 53, secured as by welding indicated at 54 to the head portion 42a, thereby preventing damage to the laminations when moving into engagement with a suitable stop or other means.

lt will be apparent from an examination of Fig. 5 of the drawings, where the electromagnet 2@ is illustrated as mounted in a position for movement along the longitudinal axis, that the bottom of the head portion 42a of the plunger 42 will tend to rest on the end of the arm 24 extending beyond extension 26. Initially, when the electromagnet is new and no wear has occurred, a -relatively small space or gap will be disposed between the upper end of the head 42a of the plunger 42 and the portion of the arm 23 beyond the extension 25. This gap is designated by the reference numeral 55 in Fig. 5 tof the drawings. It will be apparent that as the lower end of the head 42o or the arm 24 engageable therewith wears away this gap 55 will increase, thus increasing the unbalance in the forces acting on the plunger 42. Moreover, the percentage increase in unbalance with a small amount of wear is very large.

In accordance with the present invention, there are interposed between the ends of the plunger head 42a and the arms 23 and 24 suitable nonmagnetic plunger guides 57. As best shown in Figs. 6 and 7, this nonmagnetic plunger guide 57 is of U-shaped configuration, with the oight of the U secured to the arms 23 and 24 by any suitable means. Preferably the arms 23 and 24 are cut away to detine a shoulder 58 for engagement with the edge of one arm of the plunger guide 57, as clearly shown inFig. 5 of the drawings. This provides a smoothrconguration along the exterior or" the U-shaped stationary magnetic structure. The plunger guide may have a thickness of theorder of g of an inch or more, and is preferably formed of brass. or copper. It will be apparent that with the interposition of such guide 57, the equivalent of a xed air gap is provided between the lower end of the head 42a of the plunger 42 and the arm 24. This nonmagnetic gap will change very slightly with Wear due tothe wearing away of the plunger guide as well as the plunger. The effective nonmagnetic gap between the upper end of the head 42a and the arm 23 will be, of course, the thickness of the nonmagnetic occurred on one army thereof, so that the other arm of the U which is not adjacent the ends of the head 42a can be employed. Thus, there is provided an arrangement in which the disadvantages of prior art arrangeeliminated, and wherein, even occurs, the U-shaped plates 57 maybe turned around, thereby greatly increasing the life ofthe electromagnet.

It will be appreciated that the U-shaped plunger guide construction shown in Figs. 5, 6 and 7, with the bight of the U engaging one of the sides of the arms 23 and 24, is not necessary if the interchangeability of the guide is not important. Consequently', in Fig. 8 there is indicated an L-shaped plunger guide 59' which might be employed in place` of the U-shaped member S7. `(Dbviously, the guide 59 has only one leg which may be employed and cannot be turned around after wear as can the member 57. structure by the same means holding the U-shaped member 57. Addition-ally, a plate such as 69, illustrated in Fig. 9, may be employed, but suitable means for securing the same in position, such as brazing or the like, may have to be employed. in the ensuing description other modifications of the nonmagnetic plunger guide are described, and it is intended in the appended claims to cover all modifications of this guide which fall within the scope ofthe invention.

Although the electromagnet has been described as mounted in a position so that the plunger or armature moves along a longitudinal axis, it will be apparent that the electromagnet may be mounted in numerous other positions. The particular position described is the one best illustrating the importance of the nonmagnetic plunger guides of the present invention.

With an electromagnet having an extension such as 4:? at the end of the armature 42 remote from the head 42a, it may be desir-able to provide some sort of stop means to limit the maximum movement of the armature 42 away from the central pole piece 22. In accordance with the present invention, this means is incorporated with means for holding the energizing winding 34 in position. As illustrated, there is provided a four- It may be secured to the magnetic legged frame 61, having a plurality of depending legs 62fwhich may be secured. to the arms 23 andA 24 by suitable fastening means passing through the legs 62 and the arms 23 and' 24. A suitable cushioning member, such as 64, may be secured to the member 61 between the legs 62, This cushioning means may comprise a thickness of leather or similar material engageable by the plate 53 secured to the head of the plunger 42.

For the purpose of maintaining the winding 34 rmly in position with the short central pole piece 22 extending into the opening in the spool 35, there are provided a plurality of plate or retainer members 65 which are illustrated as being secured to the legs 62 as by welding or any other suitable means. These members 65 comprise two stepped portions 65a and 65h at one end thereof, as best shown in Fig. 4 of the drawings, for firmly holding the spool 35 and, consequently, the winding 34 in position. The stepped portion 6512 engages with the plate 35a of the spool 35, while the stepped portion 65a is adapted to be disposed in one of several suitable notches 67 vdefined in the edges of the plate 35a. Thus, when the frame 61 is secured in position, the stepped portion 65h of each of the plate members 65 securely clamps the spool 35, and, hence, the winding 34 against the bight portion of the U-sliapedV laminated magnetic structure of the electromagnet 20. Moreover, the stepped portions 65a of the plates 65 engage in the plurality of notches 67, thereby preventing any twisting or other motion of the spool 3S, and, hence, the winding 34, thus rmly securing the winding of the electromagnet 29 in position. The retainer members 65 also guide the sides of the plunger 42 and thus serve with the guides S7 as plunger guides.

In view of the fact that the present invention is concerned with a single phase alternating current electromagnet, suitable shading coils are provided to split the iiux and insure a displacement in time of the ilux in certain portions of the magnetic structure. As illustrated in the drawings, the plates 49 and 50 on either side of the plunger are electrically interconnected by metallic rivets such as 7i) and 71, thereby providing a oneurn electric circuit through the head of the plunger 42. This is best shown in Fig. l() of the drawings, including the ow of current induced in the one-turn shading coil. able sawcut or slot 72 is preferably provided to connect the surfaces of the plunger head 42a adjacent the rivets 70 tacting surfaces through which shaded and unshaded ilux must pass. This provides a high reluctance path for the flux to prevent it from merely being short-circuited around the rivet. Additional shading is also provided on the stationary magnet structure and comprises rivets 74, 7S, 76 and 77 which interconnect plates 28 and 29 on either sidel of the stationary magnetic structure, thereby providing a one-turn electric current path in each arm 23 and 24 of the stationary magnetic structure. Suitable sawcuts or slots 79 similar to the slots 72 are likewise provided. With this arrangement a completely chatterfree or noise-free construction is provided. Additional shading coils may be For the purpose of making sure that residual magnetism does not cause sticking, the plunger 42 is preferably provided with nonmagnetic inserts 80 which may be formed of fiber or the like and which introduce a higher relucthe end, plates 49 and 50. There is, thus, electively provided a very small nonrnagnetic gap in the portion 42h of the armature 42.

From the above description it will be apparent that there is provided an electromagnet in which the plunger 42 has three engaging surfaces with the stationary maggermano netic structure, namely, engagement ofthe end of the plunger with the center pole shoe 22, and engagement of each of the ends of the head 42a of the plunger with the extensions 25 and 26. A more efficient electromagnet is thus provided, which by virtue of the shading coils is noise-free, and by virtue of the nonmagnetic inserts 80 is free of any diculty from residual magnetism causing sticking of the armature. Likewise, long satisfactory operation is assured by virtue of the nonmagnetic plunger guides, and the increased impact surfaces (three contact surfaces).

In Figs. 11, 12 and 13 of the drawings there is illustrated an electromagnet 81 embodying a modification of the present invention insofar as the construction of the portions of the arms 23 and 24 beyond the extensions 25 and 26 is concerned. The corresponding parts of these figures are designated by the same reference numerals as in the preceding figures. Instead of having the laminations 21 extend for the entire length of the arms 23 and 24, these laminations terminate at a point shortly beyond the extensions 25 and 26, as indicated by the reference numeral 82 in Fig. 12 of the drawings. The space occupied by the laminations beyond this point in the preceding figures of the drawings is in the modification of Figs. 11 to 13 of the drawings occupied by a plurality of laminations $3 formed of nonmagnetic material which may be secured in position by suitable rivets passing through the nonmagnetic side plates 28 and 29. The laminations 83 are, therefore, sort of addendum laminations to the laminations 21 of Fig. 12 which serve the same purpose as the interposed nonmagnetic plunger guides such as 57. It will be understood that a solid piece of nonmagnetic material instead of the addendum laminations might be employed.

In Figs. 14 and 15 of the drawings there is illustrated an electromagnet, generally designated by the reference numeral 87, comprising a stationary magnetic structure designated by the reference numeral 88, which may be substantially identical with the stationary magnetic structure already described. A plunger 89, somewhat similar to the plunger 42, is provided. The plunger 89, unlike the plunger 42, does not have the extension 45, and, hence, must be connected to the means to be operated in a different manner, preferably as by connection with the head 89a thereof as by the opening 90 dened therein. For the purpose of providing the interposed nonmagnetic member between the upper and lower portions of the head 89a as viewed in Fig. 14 of the drawings, there are provided U-shaped members 91 which are similar in function to the U-shaped members 57 already described, except that the bight portions of the U-shaped members 91 engage with the ends of the arms 23 and 24 of the stationary magnetic structure 8S instead of with the side, as indicated in Fig. 5. By using U-shaped members as indicated in Fig. 14, they may be turned around so the opposite legs of the Us are effective as plunger guides, in the event that sufficient wear requires such interchange.

For the purpose of maintaining the guide members 91 in position and at the same time holding the spool and winding for the electromagnet 87, which has been omitted from Fig. 14, in position, there are provided a plurality of retaining members 92 which are L-shaped members comprising the legs 92a and 92h, as best shown in Figs. 14 and 15. The leg 92a overlies the U-shaped member 91, and, hence, maintains it in position. The leg 92b is provided with stepped portions erence numerals 65a and 65h to correspond with the same reference numerals used in the preceding figures. These stepped portions perform exactly the same functions as in the preceding figures and maintain the winding and associated spool in position. It will be understood that the L-shaped members 92 may be secured by suitable bolts or fastening means extending through the laminations, and, in eliect, extending through the arms 23 and 24 of the stationary magnetic structure. lt will also be designated by the ref.

apparent that with this arrangement the U'shaped members 91 can readily be removed and turned around so the leg thereof remote from the plunger 89 becomes the leg adjacent the plunger 89. v

In Fig. 14 there is also illustrated a modified pole shading arrangement. The end of the plunger 89 adjacent the central pole shoe 22 is provided with a shading coil comprising the rivets 95 and 96, which of course electrically interconnect the exterior plates 49 and 50 which define the plunger 89 as in the preceding embodiments. The plunger 89 is also provided with the nonmagnetic inserts 80. Likewise, the sawcuts or slots 97 are provided to connect the rivets 9S and 96 with the surface, thereby defining between the sawcuts the area of shaded ux, and outside the sawcuts the area of unshaded ux. The stationary magnetic structure is provided with a shading coil on each arm, designated by the same reference numerals as in the preceding figures, but the shading coil in the plunger head has been omitted.

It should be understood that the arrangement of Fig. 4 could employ all or part of the shading arrangement shown in Fig. 14 or Fig. 5, and, actually, in Fig. 16 a maximum of pole shading is employed. It will be appreciated that shading the plunger 89 is more etiicient than shading the stationary magnetic structure, particularly with respect to the head of the plunger, since two shading rivets, one on each leg of the stationary magnetic structure, are necessary to accomplish the same amount of shading as when the head of the plunger alone is shaded. It will, likewise, be appreciated that when the one end of the plunger 89 remote from the head 89a is shaded, either the plunger or the bight portion of the stationary magnetic structure may be shaded with the same end result.

The arrangement in Figs. 16 and 17 is substantially identical with that of Figs. 14 and 15, except that the U- shaped nonmagnetic members are replaced by nonmagnetic members 99 which have one short leg and one long leg. The arrangement functions just as does the element 91, but is not reversible in the event that wear requires reversing. Otherwise, the construction of this portion of the electromagnet, generally designated by the reference numeral 98, is identical. The shading coil of Fig. 16, which illustrates the maximum shading possible, includes in addition to the shading coils of Figs. 5 and 14 an additional shading coil comprising the rivets 100 and 101 extending through the central pole shoe 22, the ends of which are, of course, electrically connected by the metallic plates 28 and Z9 on each side of the laminations defining the stationary magnetic structure. Suitable sawcuts 102 connect the rivets with the surface of the pole shoe 22 engageable by the plunger 89 for the same purpose mentioned heretofore. It should be understood that when the expression central pole shoe or central pole piece is used in the claims it may include a portion extending from the bight of the U-shaped laminations or it may merely comprise a central portion of these laminations.

In view of the detailed description which has been included above, the operation of the electromagnet described will be readily understood by those skilled in the art. It will, moreover, be apparent that there has been provided an electromagnet which will provide more efficient operation due to eliminating any possibility of sticking of the armature. Moreover, the electromagnet designed for alternating current operation will be ex tremely quiet, and the undesirable effects of residual magnetism are reduced to a minimum.

While there have been illustrated and described nu merous embodiments of the present invention, it will be appreciated that various changes and modifications will occur to those skilled in the art, and it is aimed in the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the present invention.

Whatis claimed as new and desired to Letters'lPatent of the United States is:

l.. AnU electromagnet having a short' central pole piece and a divided laminated pole piece. on opposite sides of said central pole piece havingintegrallateral extensions directedv towards each other, a field coilsurrounding the axis of said central pole piece and having a T-shaped plunger slidable therein and between said lateral exten sions, said plunger in one extreme position making surface contact with said centralA pole piece and with both said lateral extensions, and a shading coil for shading a portion of the ux at each of the three contacting surfaces of said plunger.

2. An electromagnet having a short central pole piece and a divided laminated pole piece on opposite sides of said central pole piece having lateral extensions directed towards each other, a field coil surrounding the axis of said central pole piece and having a T-shaped plunger slidable therein and between said lateral extensions, said plunger in one extreme position engaging with said central pole piece and with both said lateral extensions, and a nonmagnetic plunger guide interposed directly between said plunger and said divided laminated pole piece on one side of said extensions.

3. An electromagnet having a stationary magnetic structure comprising a central pole piece and a divided pole piece on opposite sides of said central pole piece, an electrical winding surrounding the axis of said central pole piece, a T-shaped plunger slidable within said electrical winding, and a nonmagnetic plunger guide interposed between the head of said T-shaped plunger and a portion of said magnetic structure.

4. The electromagnet of claim 3 wherein said plunger guide is of U-shaped conguration.

5. The electromagnet of claim 3 wherein said plunger guide is U-shaped and reversible so that each leg of said U is usable as a plunger guide.

6. An electromagnet having a stationary laminated magnetic structure comprising a central pole piece and a divided laminated pole piece on opposite sides of said central pole piece having integral lateral extensions directed towards each other, a iield coil surrounding the axis of said central pole piece and having a laminated magnetic plunger slidable therein and between said lateral extensions, said plunger in one extreme position making contact at three separate surfaces, and means providing a short nonmagnetic section in said plunger whereby the eiect of residual magnetism in causing sticking of said plunger is greatly reduced.

7. An electromagnet comprising a U-shaped pole piece including a pair of lateral extensions one on each arm of said U between the end of the arm remote from the bight of said U and said bight, said lateral extensions being on adjacent sides of said arms and dening a space therebetween, an armature reciprocal within the space between said extensions and having portions engageable with said extensions and another portion engageable with the bight of said U and a nonmagnetic member interposed directly between one of said arms and said armature between said end of said arm and the associated extension.

8. An electromagnet comprising a U-shaped laminated pole piece including a pair of lateral extensions one on each arm of said U between the end of the arm remote from the bight of said U and said bight, said lateral extensions being on adjacent sides of said arms and dening a space therebetween, a laminated armature reciprocal within the space between said extensions and having two portions engageable one with each extension and another portion engageable with the bight of said U, and shading coils comprising nonmagnetic rivets extending through said pole piece and said armature.

9. An electromagnet comprising a U-shaped laminated pole piece including a pair of lateral extensions one on each arm of said U between the end of the arm remote from the bight of said U and said bight, said lateral exbe securedy by tensions being on adjacentsides ofsaid arms and dening a space therebetween, a laminated armature reciprocal within the. space betweenf saidV extensionsv and having1 por,- tions engageable with said extensions and another portion engageable with the bight of said U, and a pair of U- shaped nonmagnetic memberstsecured one to each arm on the side of said associated lateral extension remote from said bight, one arm of eachy lJ-shaped nonmagnetic member defining a guide for said armature.

10. An electromagnet comprising a U-shaped laminated pole piece including a pair of lateral extensions one on each arm of said U between the end of the arm remote from the bight of said U and said bight, said lateral extensions being on adjacent sides of said arms and defining a space therebetween, a laminated armature reciprocal within the space between said extensions and having portions engageable with said extensions and another portion engageable with the bight of said U, and a pair of nonmagnetic plate members interposed one between each of said arms and said armature to guide said armature in its reciprocal movement.

ll. An electromagnet comprising a U `shaped pole piece including a pair of lateral extensions one on each arm of said U between the end of the arm remote from the bight of said U and said bight, said lateral extensions being on adjacent sides of said arms and defining a space therebetween, an armature reciprocal within the space between said extensions and having portions engageable with said extensions and another portion engageable with the bight of said U, and nonmagnetic extensions on each of said arms defining a guide for said armature.

12. An electromagnet comprising a stationary magnetic structure including a U-shaped laminated pole piece having a pair of integral lateral extensions one on each arm of said U between the end of the arm remote from the bight of said U and said bight, said lateral extensions being on adjacent sides of said arms and defining a space therebetween, a plunger reciprocal within the space between said extensions, a winding including a spool for supporting the same surrounding the axis of said plunger and between said extensions and said bight, means for holding said spool in position comprising L-shaped members secured to said stationary magnetic structure, each including a portion forcing said spool against said bight and another portion engageable with notches in said spool, and a pair of nonmagnetic plunger guide secured one to each arm of said U, said L-shaped members retaining said plunger guides in position.

13. An electromagnet having a short central pole piece and a divided laminated pole piece on opposite sides of the axis of said central plunger slidable therein and between said lateral extensions, said plunger in one extreme position making surface contact with said central pole piece and with both said lateral extensions.

14. An electromagnet having a short central pole piece and divided laminated pole piece on opposite sides of sald central pole piece having integral lateral extensions directed towards each other, a eld coil surrounding the axis of said central pole piece and having a T-shaped plunger slidable therein and between said lateral extensions, said plunger in one extreme position making surface contact with said central pole piece and with both said lateral extensions, and a shading coil for shading a portion of the ux on at least two contacting surfaces of said plunger.

15. An electromagnet having a short central pole piece and a divided laminated pole piece on opposite sides of said central pole piece having integral lateral extensions directed towards each other, a eld coil surrounding the axis of said central pole piece and having a T-shaped plunger slidable therein and between said lateral extensions, said plunger in one extreme position making surface contact with said central pole piece and with both said lateral extensions, and shading coils for shading portions of the uxes, respectively, on selected contacting surfaces of said plunger.

References Cited in the le of this patent UNITED STATES PATENTS Coleman Jan. 30, 1912 12 l Barbarou Ian. 29, v192,4 Kendall -2-2 Ian. 29, 1929 Wi1ms Nov; 20, 1934 Kelley ....Man 21, 1939 Frese May 14, 1946- Ierey 1.- Ian. 11, 1949 Fisher Apr. 26, 1949 Sala June 6, 1950 Iencks Ian. 5, 1954 Bogue et al. Aug. 9,*-1955 

